Prior to metallization a substrate surface containing organic polymers is typically etched to achieve good adhesion between the substrate surface and plated metals. Although great efforts have been made over the years by many chemical suppliers and the plating industry to replace currently used toxic etching solutions no commercial product which is free of hexavalent chromium or Cr(VI) is currently available on the market.
Cr(VI) containing compounds are suspected of being cancer-causing. Accordingly, dealing with these compounds is subject to strict environmental regulations. In the face of the potential danger caused by Cr(VI) releasing compounds, a prohibition of the industrial use of Cr(VI) containing compounds cannot be excluded.
Over the years a variety of chemical species have been suggested in the industry for use as chrome-free oxidizing agents in wet etching processes to modify organic polymer surfaces. Such oxidants include Mn(VII), Mn(VI), Mn(IV), Ce(IV), persulfate, H2O2, organic solvents such as dioxane, metal halogenides and nitrates of Fe, Cu, Ti, Zn, and Mg. Oxidizing agents are consumed either by the etching process or due to the instability of the oxidants. Therefore frequent replenishing or a re-generation method is needed. Re-generation methods are especially preferred for industrial settings. Mn(VII) is one of the most commonly used oxidizing agents. In solution it is typically in the form of ionic species MnO4−. Electrochemical regeneration of Mn(VII) in alkaline solutions has been used in different industries, such as in the manufacture of circuit boards. In acidic media re-generation of Mn(VII) appears more difficult than in an alkaline medium. Published literature using catalysts such as Ag(I) or Bi(III) for electrochemical oxidation is sparse. The studies of Fleischmann et al. (J. Appl. Electrochem. Vol. 1, pp. 1, 1971) have shown that Ag(I) is a good catalyst for oxidizing both organic and inorganic species electrochemically. Park et al. (J. Electrochem. Soc. Vol. 151, pp. E265, 2004) disclose that Bi(III) on a boron-doped diamond (BDD) electrode also may act as electron transfer mediator to oxidize Mn(II) to Mn(VII). Boardman (J. Electroanal. Chem. Vol. 149, pp. 49, 1983) and Comninellis (Electrochimica Acta, Vol. 36, No. 8, pp. 1363, 1991) have demonstrated the possibility of electrochemical formation of Mn(VII) from Mn(II) in a sulfuric acid medium in the presence of Ag(I) and the experimental conditions to increase the current efficiency of Mn(VII) formation. US2011/0140035 discloses similar methods for use in a permanganate acidic pickling solution for pre-treatment of plastic surfaces.
However, either in an acidic or alkaline medium, Mn(VII) is unstable and tends to reduce to its lower oxidation states, especially to Mn(IV), consequently forming a large quantity of the insoluble MnO2 and causing quality issues on the treated polymer surfaces. Frequent removal of MnO2 precipitation from the permanganate etching solution is therefore required in the industrial scale operation. Therefore, there is a need for a method which does not form substantial amounts of undesirable insoluble reaction products and the oxidizing agent can be re-generated during continuous operation.